Angularly Resolved Scatterometer and Inspection Method

ABSTRACT

An inspection method is provided to determine a value related to a parameter of a target pattern printed on a substrate by a lithographic process used to manufacture a device layer on a substrate. The inspection method can include using an optical system with a high-NA objective lens, where the high-NA objective lens includes an object plane and a pupil plane. The inspection method can also include providing an aperture member to define at least one obscuration, determining a radial distance between a radially innermost point of each dark area and a nominal center of an image in a pupil plane, and determining an axial distance between the target and an object plane from the determined radial distance.

CROSS REFERENCE TO RELATED APPLICATION

This application is a divisional application of U.S. patent applicationSer. No. 11/785,426, filed Apr. 17, 2007 (now allowed), entitled“Angularly Resolved Scatterometer and Inspection Method,” which isincorporated herein by reference in its entirety.

BACKGROUND

1. Field

The present invention relates to methods of inspection usable, forexample, in the manufacture of devices by lithographic techniques and tomethods of manufacturing devices using lithographic techniques.

2. Related Art

A lithographic apparatus is a machine that applies a desired patternonto a substrate, usually onto a target portion of the substrate. Alithographic apparatus can be used, for example, in the manufacture ofintegrated circuits (ICs). In that instance, a patterning device, whichis alternatively referred to as a mask or a reticle, may be used togenerate a circuit pattern to be formed on an individual layer of theIC. This pattern can be transferred onto a target portion (e.g.including part of, one, or several dies) on a substrate (e.g. a siliconwafer). Transfer of the pattern is typically via imaging onto a layer ofradiation-sensitive material (resist) provided on the substrate. Ingeneral, a single substrate will contain a network of adjacent targetportions that are successively patterned. Known lithographic apparatusinclude so-called steppers, in which each target portion is irradiatedby exposing an entire pattern onto the target portion at one time, andso-called scanners, in which each target portion is irradiated byscanning the pattern through a radiation beam in a given direction (the“scanning”-direction) while synchronously scanning the substrateparallel or anti-parallel to this direction. It is also possible totransfer the pattern from the patterning device to the substrate byimprinting the pattern onto the substrate.

In order to monitor the lithographic process, it is desirable to measureparameters of the patterned substrate, for example the overlay errorbetween successive layers formed in or on it. There are varioustechniques for making measurements of the microscopic structures formedin lithographic processes, including the use of scanning electronmicroscopes and various specialized tools. One form of specializedinspection tool is a scatterometer in which a beam of radiation isdirected onto a target on the surface of the substrate and properties ofthe scattered or reflected beam are measured. By comparing theproperties of the beam before and after it has been reflected orscattered by the substrate, the properties of the substrate can bedetermined. This can be done, for example, by comparing the reflectedbeam with data stored in a library of known measurements associated withknown substrate properties. Two main types of scatterometer are known.Spectroscopic scatterometers direct a broadband radiation beam onto thesubstrate and measure the spectrum (intensity as a function ofwavelength) of the radiation scattered into a particular narrow angularrange. Angularly resolved scatterometers use a monochromatic radiationbeam and measure the intensity of the scattered radiation as a functionof angle.

In an angularly resolved scatterometer, the target being measured or afiducial used for calibration or normalization is in focus. To this end,an optical, e.g. a Foucault knife edge, or capacitive focus sensor maybe provided. However, when using such a sensor, small focus errors(defocus) may remain, e.g. due to process effects related to thestructure on the substrate being measured or due to settling time. Suchsmall residual defocus should not, in theory, lead to measurement errorsin this type of scatterometer. However, the present inventor hasdetermined that residual defocus does cause measurement errors.

SUMMARY

It is desirable to provide an angularly resolved scatterometer andscatterometry method that do not exhibit, or exhibit to a lesser extent,measurement errors due to residual defocus.

According to an embodiment of the invention, there is provided aninspection method to determine a value related to a parameter of atarget pattern printed on a substrate by a lithographic process used tomanufacture a device layer on a substrate, the method including: usingan optical system including a high-NA objective lens having an objectplane and a pupil plane to direct a first beam of radiation on to thetarget pattern, to collect radiation reflected or scattered by thetarget pattern and to project a second beam of radiation to form animage of the pupil plane of the objective lens in an image plane;providing an aperture member in the path of the second beam at alocation not congruent with the pupil plane of the objective lens, theaperture member defining at least one obscuration extending apredetermined distance into the second beam so as to form a dark area inthe image of the pupil plane; determining a radial distance between aradially innermost point of the or each dark area and a nominal centerof the image of the pupil plane; and determining an axial distancebetween the target and the object plane from the determined radialdistance(s).

According to an embodiment of the invention, there is provided aninspection method to determine a value related to a parameter of atarget pattern printed on a substrate by a lithographic process used tomanufacture a device layer on a substrate, the method including: usingan optical system including a high-NA objective lens having an objectplane and a pupil plane to direct a first beam of radiation on to areference member, to collect radiation reflected or scattered by thereference member and to project a second beam of radiation to form animage of the pupil plane of the objective lens in an image plane;relatively moving the reference member and the optical system in thedirection substantially perpendicular to the object plane so as toposition the reference member at a plurality of positions havingdifferent distances from the object plane; when the reference member ispositioned at each of the plurality of positions, capturing ascatterometric spectra of the reference member; storing thescatterometric spectra of the reference member as a plurality ofnormalization spectra; using the optical system to direct the first beamof radiation on to the target pattern, to collect radiation reflected orscattered by the target pattern and to project a second beam ofradiation to form an image of the pupil plane of the objective lens inan image plane; capturing a scatterometric spectra of the targetpattern; determining the distance between the target pattern and theobject plane; obtaining a normalization spectrum based on the determineddistance between the target pattern and the object plane; normalizingthe spectrum of the target pattern using the obtained normalizationspectrum to obtain a normalized spectrum; and determining the valuerelated to the parameter from the normalized spectrum.

According to an embodiment of the invention, there is provided aninspection method using a scatterometer to determine a value related toa parameter of a target pattern printed on a substrate by a lithographicprocess used to manufacture a device layer on a substrate, the methodincluding: obtaining a plurality of normalization spectra using areference member in the scatterometer in place of the substrate, thenormalization spectra being obtained with the reference memberpositioned at various different defocus values; obtaining a measurementspectrum for the target pattern using the scatterometer; determining thedefocus value at the time the measurement spectrum was obtained;normalizing the measurement spectrum using a normalization spectrumcorresponding to the determined defocus value to obtain a normalizedspectrum; determining the value related to a parameter from thenormalized spectrum.

According to an embodiment of the invention, there is provided aninspection method using a scatterometer to determine a value related toa parameter of a target pattern printed on a substrate by a lithographicprocess used to manufacture a device layer on a substrate, thescatterometer including an optical system including a high-NA objectivelens having an object plane and a pupil plane to direct a first beam ofradiation on to the target pattern, to collect radiation reflected orscattered by the target pattern and to project a second beam ofradiation to form an image of the pupil plane of the objective lens inan image plane, the method including: obtaining a plurality ofnormalization spectra using a reference member in the scatterometer inplace of the substrate, the normalization spectra being obtained withthe reference member positioned at various different defocus values;providing an aperture member in the path of the second beam at alocation not congruent with the pupil plane of the objective lens, theaperture member defining at least one obscuration extending apredetermined distance into the second beam so as to form a dark area inthe image of the pupil plane; obtaining a measurement spectrum for thetarget pattern using the scatterometer; determining a radial distancebetween a radially innermost point of the or each dark area and anominal center of the image of the pupil plane; determining a defocusvalue, being an axial distance between the target and the object planefrom the determined radial distance(s); normalizing the measurementspectrum using a normalization spectrum corresponding to the determineddefocus value to obtain a normalized spectrum; and determining the valuerelated to a parameter from the normalized spectrum.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of exampleonly, with reference to the accompanying schematic drawings in whichcorresponding reference symbols indicate corresponding parts, and inwhich:

FIG. 1 depicts a lithographic apparatus in accordance with an embodimentof the invention;

FIG. 2 depicts a lithographic cell or cluster in accordance with anembodiment of the invention;

FIG. 3 depicts a scatterometer according to an embodiment of theinvention;

FIG. 4 depicts an aperture plate;

FIG. 5 depicts an image of the aperture plate of FIG. 4 in the detectorof the scatterometer of FIG. 3; and

FIG. 6 is a flow chart depicting procedures in a method according to anembodiment of the invention.

DETAILED DESCRIPTION

FIG. 1 schematically depicts a lithographic apparatus. The apparatusincludes: an illumination system (illuminator) IL configured tocondition a radiation beam B (e.g. UV radiation or DUV radiation); asupport structure or pattern support (e.g. a mask table) MT constructedto support a patterning device (e.g. a mask) MA and connected to a firstpositioner PM configured to accurately position the patterning device inaccordance with certain parameters; a substrate table (e.g. a wafertable) WT constructed to hold a substrate (e.g. a resist-coated wafer) Wand connected to a second positioner PW configured to accuratelyposition the substrate in accordance with certain parameters; and aprojection system (e.g. a refractive projection lens system) PLconfigured to project a pattern imparted to the radiation beam B bypatterning device MA onto a target portion C (e.g. including one or moredies) of the substrate W.

The illumination system may include various types of optical components,such as refractive, reflective, magnetic, electromagnetic, electrostaticor other types of optical components, or any combination thereof, fordirecting, shaping, or controlling radiation.

The support structure supports, i.e. bears the weight of, the patterningdevice. It holds the patterning device in a manner that depends on theorientation of the patterning device, the design of the lithographicapparatus, and other conditions, such as for example whether or not thepatterning device is held in a vacuum environment. The support structurecan use mechanical, vacuum, electrostatic or other clamping techniquesto hold the patterning device. The support structure may be a frame or atable, for example, which may be fixed or movable as required. Thesupport structure may ensure that the patterning device is at a desiredposition, for example with respect to the projection system. Any use ofthe terms “reticle” or “mask” herein may be considered synonymous withthe more general term “patterning device.”

The term “patterning device” used herein should be broadly interpretedas referring to any device that can be used to impart a radiation beamwith a pattern in its cross-section such as to create a pattern in atarget portion of the substrate. It should be noted that the patternimparted to the radiation beam may not exactly correspond to the desiredpattern in the target portion of the substrate, for example if thepattern includes phase-shifting features or so called assist features.Generally, the pattern imparted to the radiation beam will correspond toa particular functional layer in a device being created in the targetportion, such as an integrated circuit.

The patterning device may be transmissive or reflective. Examples ofpatterning devices include masks, programmable mirror arrays, andprogrammable LCD panels. Masks are well known in lithography, andinclude mask types such as binary, alternating phase-shift, andattenuated phase-shift, as well as various hybrid mask types. An exampleof a programmable mirror array employs a matrix arrangement of smallmirrors, each of which can be individually tilted so as to reflect anincoming radiation beam in different directions. The tilted mirrorsimpart a pattern in a radiation beam, which is reflected by the mirrormatrix.

The term “projection system” used herein should be broadly interpretedas encompassing any type of projection system, including refractive,reflective, catadioptric, magnetic, electromagnetic and electrostaticoptical systems, or any combination thereof, as appropriate for theexposure radiation being used, or for other factors such as the use ofan immersion liquid or the use of a vacuum. Any use of the term“projection lens” herein may be considered as synonymous with the moregeneral term “projection system”.

As here depicted, the apparatus is of a transmissive type (e.g.employing a transmissive mask). Alternatively, the apparatus may be of areflective type (e.g. employing a programmable mirror array of a type asreferred to above, or employing a reflective mask).

The lithographic apparatus may be of a type having two (dual stage) ormore substrate tables (and/or two or more mask tables). In such“multiple stage” machines the additional tables may be used in parallel,or preparatory steps may be carried out on one or more tables while oneor more other tables are being used for exposure.

The lithographic apparatus may also be of a type wherein at least aportion of the substrate may be covered by a liquid having a relativelyhigh refractive index, e.g. water, so as to fill a space between theprojection system and the substrate. An immersion liquid may also beapplied to other spaces in the lithographic apparatus, for example,between the patterning device (e.g. mask) and the projection system.Immersion techniques are well known in the art for increasing thenumerical aperture of projection systems. The term “immersion” as usedherein does not mean that a structure, such as a substrate, must besubmerged in liquid, but rather only means that liquid is locatedbetween the projection system and the substrate during exposure.

Referring to FIG. 1, the illuminator IL receives a radiation beam from aradiation source SO. The source and the lithographic apparatus may beseparate entities, for example when the source is an excimer laser. Insuch cases, the source is not considered to form part of thelithographic apparatus and the radiation beam is passed from the sourceSO to the illuminator IL with the aid of a beam delivery system BDincluding, for example, suitable directing mirrors and/or a beamexpander. In other cases the source may be an integral part of thelithographic apparatus, for example when the source is a mercury lamp.The source SO and the illuminator IL, together with the beam deliverysystem BD if required, may be referred to as a radiation system.

The illuminator IL may include an adjuster AD for adjusting the angularintensity distribution of the radiation beam. Generally, at least theouter and/or inner radial extent (commonly referred to as σ-outer andσ-inner, respectively) of the intensity distribution in a pupil plane ofthe illuminator can be adjusted. In addition, the illuminator IL mayinclude various other components, such as an integrator IN and acondenser CO. The illuminator may be used to condition the radiationbeam, to have a desired uniformity and intensity distribution in itscross-section.

The radiation beam B is incident on the patterning device (e.g., maskMA), which is held on the support structure (e.g., mask table) MT, andis patterned by the patterning device. Having traversed the patterningdevice (e.g. mask) MA, the radiation beam B passes through theprojection system PL, which focuses the beam onto a target portion C ofthe substrate W. With the aid of the second positioner PW and positionsensor IF (e.g. an interferometric device, linear encoder, 2-D encoderor capacitive sensor), the substrate table WT can be moved accurately,e.g. so as to position different target portions C in the path of theradiation beam B. Similarly, the first positioner PM and anotherposition sensor (which is not explicitly depicted in FIG. 1 a) can beused to accurately position the patterning device (e.g. mask) MA withrespect to the path of the radiation beam B, e.g. after mechanicalretrieval from a mask library, or during a scan. In general, movement ofthe mask table MT may be realized with the aid of a long-stroke module(coarse positioning) and a short-stroke module (fine positioning), whichform part of the first positioner PM. Similarly, movement of thesubstrate table WT may be realized using a long-stroke module and ashort-stroke module, which form part of the second positioner PW. In thecase of a stepper (as opposed to a scanner) the support structure (e.g.mask table) MT may be connected to a short-stroke actuator only, or maybe fixed. Patterning device (e.g. mask) MA and substrate W may bealigned using mask alignment marks M1, M2 and substrate alignment marksP1, P2. Although the substrate alignment marks as illustrated occupydedicated target portions, they may be located in spaces between targetportions (these are known as scribe-lane alignment marks). Similarly, insituations in which more than one die is provided on the patterningdevice (e.g. mask) MA, the mask alignment marks may be located betweenthe dies.

The depicted apparatus could be used in at least one of the followingmodes:

1. In step mode, the support structure or pattern support (e.g. masktable) MT and the substrate table WT are kept essentially stationary,while an entire pattern imparted to the radiation beam is projected ontoa target portion C at one time (i.e. a single static exposure). Thesubstrate table WT is then shifted in the X and/or Y direction so that adifferent target portion C can be exposed. In step mode, the maximumsize of the exposure field limits the size of the target portion Cimaged in a single static exposure.

2. In scan mode, the support structure or pattern support (e.g. masktable) MT and the substrate table WT are scanned synchronously while apattern imparted to the radiation beam is projected onto a targetportion C (i.e. a single dynamic exposure). The velocity and directionof the substrate table WT relative to the support structure (e.g. masktable) MT may be determined by the (de-)magnification and image reversalcharacteristics of the projection system PL. In scan mode, the maximumsize of the exposure field limits the width (in the non-scanningdirection) of the target portion in a single dynamic exposure, whereasthe length of the scanning motion determines the height (in the scanningdirection) of the target portion.

3. In another mode, the support structure (e.g. mask table) MT is keptessentially stationary holding a programmable patterning device, and thesubstrate table WT is moved or scanned while a pattern imparted to theradiation beam is projected onto a target portion C. In this mode,generally a pulsed radiation source is employed and the programmablepatterning device is updated as required after each movement of thesubstrate table WT or in between successive radiation pulses during ascan. This mode of operation can be readily applied to masklesslithography that utilizes programmable patterning device, such as aprogrammable mirror array of a type as referred to above.

Combinations and/or variations on the above described modes of use orentirely different modes of use may also be employed.

As shown in FIG. 2, the lithographic apparatus LA forms part of alithographic cell LC, also sometimes referred to a lithocell or cluster,which also includes apparatus to perform pre- and post-exposureprocesses on a substrate. Conventionally these include spin coaters SCto deposit resist layers, developers DE to develop exposed resist, chillplates CH and bake plates BK. A substrate handler, or robot, RO picks upsubstrates from input/output ports I/O1, I/O2, moves them between thedifferent process apparatus and delivers then to the loading bay LB ofthe lithographic apparatus. These devices, which are often collectivelyreferred to as the track, are under the control of a track control unitTCU which is itself controlled by the supervisory control system SCS,which also controls the lithographic apparatus via lithography controlunit LACU. Thus, the different apparatus can be operated to maximizethroughput and processing efficiency.

In order that the substrates that are exposed by the lithographicapparatus are exposed correctly and consistently, it is desirable toinspect exposed substrates to measure properties such as overlay errorsbetween subsequent layers, line thicknesses, critical dimensions (CD),etc. If errors are detected, adjustments may be made to exposures ofsubsequent substrates, especially if the inspection can be done soon andfast enough that other substrates of the same batch are still to beexposed. Also, already exposed substrates may be stripped andreworked—to improve yield—or discarded—thereby avoiding performingexposures on substrates that are known to be faulty. In a case whereonly some target portions of a substrate are faulty, further exposurescan be performed only on those target portions which are good.

An inspection apparatus is used to determine the properties of thesubstrates, and in particular, how the properties of differentsubstrates or different layers of the same substrate vary from layer tolayer. The inspection apparatus may be integrated into the lithographicapparatus LA or the lithocell LC or may be a stand-alone device. Toenable most rapid measurements, it is desirable that the inspectionapparatus measure properties in the exposed resist layer immediatelyafter the exposure. However, the latent image in the resist has a verylow contrast—there is only a very small difference in refractive indexbetween the parts of the resist which have been exposed to radiation andthose which have not—and not all inspection apparatus have sufficientsensitivity to make useful measurements of the latent image. Thereforemeasurements may be taken after the post-exposure bake step (PEB) whichis customarily the first step carried out on exposed substrates andincreases the contrast between exposed and unexposed parts of theresist. At this stage, the image in the resist may be referred to assemi-latent. It is also possible to make measurements of the developedresist image—at which point either the exposed or unexposed parts of theresist have been removed—or after a pattern transfer step such asetching. The latter possibility limits the possibilities for rework offaulty substrates but may still provide useful information.

A scatterometer SM2 according to an embodiment of the present inventionis shown in FIG. 3. In this device, the radiation emitted by radiationsource unit 2 is collimated using lens system 12 through polarizer 17,reflected by partially reflected surface 16 and is focused ontosubstrate W via a microscope objective lens 15, which has a highnumerical aperture (NA), preferably at least 0.9 and more preferably atleast 0.95. Immersion scatterometers may even have lenses with numericalapertures over 1. The reflected radiation then transmits throughpartially reflective surface 16 into a detector 18 in order to have thescatter spectrum detected. The detector may be located in theback-projected pupil plane 11, which is at the focal length of the lenssystem 15, however the pupil plane may instead be re-imaged withauxiliary optics (not shown) onto the detector. The pupil plane is aplane in which the radial position of radiation defines the angle ofincidence and the angular position defines azimuth angle of theradiation. The detector is preferably a two-dimensional detector so thata two-dimensional angular scatter spectrum of the substrate target canbe measured. The detector 18 may be, for example, an array of CCD orCMOS sensors, and may use an integration time of, for example, 40milliseconds per frame.

A reference beam is often used for example to measure the intensity ofthe incident radiation. To do this, when the radiation beam is incidenton the beam splitter 16 part of it is transmitted through the beamsplitter as a reference beam towards a reference mirror 14. Thereference beam is then projected onto a different part of the samedetector 18.

The detector 18 may measure the intensity of scattered light at a singlewavelength (or narrow wavelength range), the intensity separately atmultiple wavelengths or integrated over a wavelength range. Furthermore,the detector may separately measure the intensity of transversemagnetic- and transverse electric-polarized light and/or the phasedifference between the transverse magnetic- and transverseelectric-polarized light.

The target on substrate W may be a grating, which is printed such thatafter development, the bars are formed of solid resist lines. The barsmay alternatively be etched into the substrate. This pattern issensitive to aberrations in the lithographic projection apparatus,particularly the projection system PL, and illumination symmetry and thepresence of such aberrations will manifest themselves in a variation inthe printed grating. Accordingly, the scatterometry data of the printedgratings is used to reconstruct the gratings. The parameters of thegrating, such as line widths and shapes, may be input to thereconstruction process, performed by processing unit PU, from knowledgeof the printing step and/or other scatterometry processes. Other formsof target may be used to measure other parameters of structures on thesubstrate or the processes used to produce them.

The present inventor has determined that in an angularly resolvedscatterometer, measurement errors due to residual defocus can be causedby dirt and/or imperfections in the optical elements in the measurementbranch (that is the optical path from target to detector) of thescatterometer. In particular, the errors in the recorded spectra due todirt and imperfections increase towards the outer edge of the pupilplane, which is where much of the information used for measurements isoften to be found. To address this problem the present embodimentemploys a novel focus error detection arrangement and employs a novelerror compensation method which are described below in turn. Althoughthe focus error detection arrangement and the focus error compensationmethod may be used independently to advantage, a particularly beneficialeffect is obtained when they are used together.

For focus error detection in the embodiment of the invention, aparticular aperture member 19 is placed in the measurement branch at alocation optically away from the pupil plane, e.g. after optics thatre-image the pupil plane on the sensor. The aperture member 12 ispositioned in a path of the beam of radiation at a location notcongruent with the pupil plane of the lens system. The aperture memberis shown in FIG. 4 and has a central transparent region 20 with adiameter larger than the nominal pupil diameter. One or moreobscurations 21 project inwardly so that they are visible in the pupilimage recorded by the detector 18. In a preferred embodiment theobscurations are substantially opaque, but they may be partiallytransmissive as long as they are sufficiently opaque to form discernibleshadows on the detector. In the figure, four triangular obscurations areshown at azimuth angles of about +/−45° and about +/−135° but theobscurations do not need to be triangular and do not need to be at thesepositions. Other than that they are visible in the pupil image and theposition of the image detectably changes with defocus, the onlyrequirements on the shape and position of the obscurations is that theydo not obscure too much useful information in the pupil image. Aparticularly preferred form of obscuration is a set of lines or arcsparallel to the edge of the pupil or the tangent to edge of the pupil sothat a grating is formed on the detector. Radial shifts in this gratingcan be measured very accurately by comparing the image of the shadowwith a reference image to form a phase grating. Multiple, azimuthallyspaced obscurations allow elimination of tilt effects from themeasurement of defocus by differential measurements. Additionalobscurations are useful to allow averaging to reduce errors. Theaperture member may be provided with an actuator (not shown) to enableit to be selectively removed from the beam path.

FIG. 5 shows the image of the aperture 19 on the detector 18. The imageof the obscurations 21 is blurred and the radial position r of theirends (radially innermost points), or another discernable point on eachimage, depends on the amount of defocus of the substrate or fiducialbeing measured. The distance r can be determined by an image recognitionalgorithm executed by the processing unit PU. By appropriatecalibration, for example, a relationship between r and defocus can beobtained. Having determined the defocus value, appropriate correctionscan be made, e.g. by adjusting the position of the substrate or fiducialbeing measured. The above described method of determining defocus hasthe benefits that it is accurate and can be performed quickly so thatthere is no loss of throughput. Furthermore, the measurement of defocuscan be obtained off-line from images of the pupil plane captured to makemeasurements on the target so that the defocus values are exactlycontemporaneous with the parameter measurements, avoiding all data agingissues.

A novel method of compensating for defocus errors will now be describedwith reference to FIG. 6. The first procedure, S1, is to capture a setof images of a blank fiducial (e.g. an aluminum plate with low surfaceroughness) at a variety of different defocus values, both positive andnegative, that are expected to occur in use. This can be done by movingthe fiducial up and down as necessary. The necessary number of imagescan be captured, for example, during a substrate exchange or betweenlots without loss of throughput. Depending on the stability of theoptical system, it may not be necessary to capture a set of images thatfrequently. If multiple wavelengths and/or polarization states are to beused for capturing measurement spectra, for maximum accuracy a set ofnormalization spectra is obtained for each wavelength and/orpolarization state. The captured images and related defocus values arestored in a database as a set of normalization spectra for later use.Images for defocus values between those at which images were capturedcan be interpolated either in advance or at the time of use. Thenormalization spectra measure the effect of dirt and/or imperfections inthe measurement branch of the optical system.

The target, a parameter of which is to be determined, is measured inprocedure S2 to obtain a measurement image or spectra, in theconventional manner. Before processing the measurement spectra to obtainthe parameter of interest, the defocus values at the time of themeasurements are determined S3, e.g. by the method using aperture 19described above. Next, in procedure S4, a suitable normalizationspectrum is obtained from the database, or interpolated from storedspectra. In procedure S5, the measurement spectra are divided by theselected or calculated normalization spectrum to obtain a normalizedspectrum. In procedure S6, the normalized spectra is processed todetermine the parameter of interest. This can be done in any suitablemanner, known to the person skilled in the art, such as by rigorouscoupled wave analysis (RCWA), library search of pre-measured orsimulated spectra, iterative methods or principal component analysis(PCA). It is then determined S7 whether there are more targets tomeasure in which case procedures S2 to S6 are repeated as often asnecessary. In general procedure S1, measurement of the fiducial atmultiple defocus values, is only repeated no more than once persubstrate or batch and in many cases once per day or less frequentlywill suffice. However, if for any reason, the defocus errors change overa very short timescale, procedure S1 might be repeated each measurementtarget.

A particular benefit when the above method of compensating for defocusis combined with the above described method of determining defocus isthat procedure S3 can be performed from the image captured formeasurement purposes and so procedures S3 to S6 can all be carried outoff-line and/or in parallel with the acquisition of spectra from othertargets so there is no loss of throughput. The fact that the residualdefocus can be compensated for by the above described method means thatno additional procedures, such as adjusting the position of thesubstrate relative to the objective lens, need to be taken at the timeof image capture.

Although specific reference may be made in this text to the use oflithographic apparatus in the manufacture of ICs, it should beunderstood that the lithographic apparatus described herein may haveother applications, such as the manufacture of integrated opticalsystems, guidance and detection patterns for magnetic domain memories,flat-panel displays, liquid-crystal displays (LCDs), thin film magneticheads, etc. The skilled artisan will appreciate that, in the context ofsuch alternative applications, any use of the terms “wafer” or “die”herein may be considered as synonymous with the more general terms“substrate” or “target portion”, respectively. The substrate referred toherein may be processed, before or after exposure, in for example atrack (a tool that typically applies a layer of resist to a substrateand develops the exposed resist), a metrology tool and/or an inspectiontool. Where applicable, the disclosure herein may be applied to such andother substrate processing tools. Further, the substrate may beprocessed more than once, for example in order to create a multi-layerIC, so that the term substrate used herein may also refer to a substratethat already contains multiple processed layers.

Although specific reference may have been made above to the use ofembodiments of the invention in the context of optical lithography, itwill be appreciated that the invention may be used in otherapplications, for example imprint lithography, and where the contextallows, is not limited to optical lithography. In imprint lithography atopography in a patterning device defines the pattern created on asubstrate. The topography of the patterning device may be pressed into alayer of resist supplied to the substrate whereupon the resist is curedby applying electromagnetic radiation, heat, pressure or a combinationthereof. The patterning device is moved out of the resist leaving apattern in it after the resist is cured.

The terms “radiation” and “beam” used herein encompass all types ofelectromagnetic radiation, including ultraviolet (UV) radiation (e.g.having a wavelength of or about 365, 355, 248, 193, 157 or 126 nm) andextreme ultra-violet (EUV) radiation (e.g. having a wavelength in therange of 5-20 nm), as well as particle beams, such as ion beams orelectron beams.

The term “lens”, where the context allows, may refer to any one orcombination of various types of optical components, includingrefractive, reflective, magnetic, electromagnetic and electrostaticoptical components.

While specific embodiments of the invention have been described above,it will be appreciated that the invention may be practiced otherwisethan as described. For example, the invention may take the form of acomputer program containing one or more sequences of machine-readableinstructions describing a method as disclosed above, or a data storagemedium (e.g. semiconductor memory, magnetic or optical disk) having sucha computer program stored therein.

The descriptions above are intended to be illustrative, not limiting.Thus, it will be apparent to one skilled in the art that modificationsmay be made to the invention as described without departing from thescope of the claims set out below.

1. A method comprising: using an optical system comprising an objectivelens including an object plane and a pupil plane to direct a first beamof radiation onto a target pattern on a substrate, to collect radiationreflected or scattered by the target pattern, and to project a secondbeam of radiation to form an image of the pupil plane of the objectivelens in an image plane; using an aperture to define at least oneobscuration extending a predetermined distance into the second beam soas to form a dark area in the image of the pupil plane, the aperturebeing in a path of the second beam at a location not congruent with thepupil plane of the objective lens; determining a radial distance betweena radially innermost point of the dark area and a nominal center of theimage of the pupil plane; and determining an axial distance between thetarget and the object plane from the determined radial distance.
 2. Themethod of claim 1, further comprising: using the optical system todirect the first beam of radiation onto a second target pattern, tocollect radiation reflected or scattered by the second target pattern,and to project the second beam of radiation to form a second image ofthe pupil plane of the objective lens in the image plane in parallelwith the determining of the radial distance and the determining of theaxial distance.
 3. A method comprising: using an optical systemcomprising an objective lens including an object plane and a pupil planeto direct a first beam of radiation onto a reference member, to collectradiation reflected or scattered by the reference member, and to projecta second beam of radiation to form an image of the pupil plane of theobjective lens in an image plane; relatively moving the reference memberand the optical system in a direction substantially perpendicular to theobject plane so as to position the reference member at a plurality ofpositions having different distances from the object plane; capturingscatterometric spectra of the reference member when the reference memberis positioned at each of the plurality of positions; storing thescatterometric spectra of the reference member as a plurality ofnormalization spectra; using the optical system to direct the first beamof radiation onto a target pattern, to collect radiation reflected orscattered by the target pattern, and to project a second beam ofradiation to form a second image of the pupil plane of the objectivelens in the image plane; capturing scatterometric spectra of the targetpattern; determining the distance between the target pattern and theobject plane; obtaining a normalization spectrum based on the determineddistance between the target pattern and the object plane; normalizingthe spectrum of the target pattern using the obtained normalizationspectrum to obtain a normalized spectrum; and determining a valuerelated to a parameter of the target pattern from the normalizedspectrum.
 4. The method of claim 3, wherein the obtaining thenormalization spectrum comprises storing the normalization spectra andselecting one of the stored normalization spectra.
 5. The method ofclaim 3, wherein the obtaining the normalization spectrum comprisesstoring the normalization spectra and interpolating between a pluralityof the stored normalization spectra.
 6. The method of claim 3, whereinthe plurality of positions comprises at least one of a position wherethe reference member is between the optical system and the object planeand a position where the reference member is on the other side of theobject plane in the optical system.
 7. A method comprising: obtaining aplurality of normalization spectra using a reference member in ascatterometer, the normalization spectra being obtained with thereference member positioned at various different defocus values;obtaining a measurement spectrum for a target pattern using thescatterometer; determining a defocus value at a time the measurementspectrum was obtained; normalizing the measurement spectrum using anormalization spectrum corresponding to the determined defocus value toobtain a normalized spectrum; and determining a value related to aparameter of the target pattern from the normalized spectrum.
 8. Themethod of claim 7, further comprising repeating the obtaining themeasurement spectrum, the determining the defocus value, the normalizingthe measurement spectrum, and the determining the value related to theparameter for a plurality of target patterns before repeating theobtaining the plurality of normalization spectra.
 9. The method of claim8, wherein the repeated obtaining the measurement spectrum is performedin parallel with the determining the defocus value, the normalizing themeasurement spectrum, and the determining the value related to theparameter for a previous target pattern.
 10. An inspection method usinga scatterometer to determine a value related to a parameter of a targetpattern printed on a substrate by a lithographic process used tomanufacture a device layer on a substrate, the scatterometer comprisingan optical system including a high-NA objective lens including an objectplane and a pupil plane to direct a first beam of radiation onto thetarget pattern, to collect radiation reflected or scattered by thetarget pattern, and to project a second beam of radiation to form animage of the pupil plane of the objective lens in an image plane, themethod comprising: obtaining a plurality of normalization spectra usinga reference member in the scatterometer in place of the substrate, thenormalization spectra being obtained with the reference memberpositioned at various different defocus values; providing an aperturemember in a path of the second beam of radiation at a location notcongruent with the pupil plane of the objective lens, the aperturemember defining at least one obscuration extending a predetermineddistance into the second beam of radiation so as to form a dark area inthe image of the pupil plane; obtaining a measurement spectrum for thetarget pattern using the scatterometer; determining a radial distancebetween a radially innermost point of the dark area and a nominal centerof the image of the pupil plane; determining a defocus value, thedefocus value comprising an axial distance between the target and theobject plane from the determined radial distance; normalizing themeasurement spectrum using a normalization spectrum corresponding to thedetermined defocus value to obtain a normalized spectrum; anddetermining the value related to the parameter of the target patternfrom the normalized spectrum.